Saturday, August 01, 2009

A Test Client for App Engine

I created a simple utility library in Python to help anyone debug their App Engine application. Many of the App Engine apps that I've seen use HTTP, HTML form posts, and the Users API and the easiest way to test these features is to fire up the web browser and click through the web pages generated by the app. However this can be a bit slow and repetative and it is difficult to determine exactly what is being sent over the wire (though this is greatly helped by using wireshark, fiddler, tcpdump, or antoher network packet sniffing tool).

Enter my little App Engine HTTP module. It provides a simple interface for making arbitrary HTTP requests and will print the full request and response to the terminal (though you can turn the noisy printing off if you want). Download, copy the http.py file to you working directory and try it out in your Python interpreter.

For our first demonstration, let's try to visit the Google search page.

You should see your request and the server's response (with the HTML for the Google Search page) in your terminal window. This should work with just about any website out there.

Other HTTP debugging tools can show you the request and response like this, but I find that this kind of simple Python client can be useful in writing end-to-end or integration tests which contact your App Engine app remotely.

Along those lines, one of the things which standard HTTP debugging tools do not provide, is a way to sign in to an App Engine app with a Google Account so that the App Engine Users API can identify the current user. I wrote an extremely simple app which illustrates the Users API, try it out here:

After signing in, the page should simply say, "Hello yourusername (yourusername@yourdomain.com)" You'll notice that during the sign in process, you signed in on www.google.com/accounts and were asked to approve access to the app. This kind of interaction works great in a browser, but can be tricky when you are using a command line, browserless, client.

It is possible however, to sign in to an App Engine app without using a browser. You can use the same technique used in appcfg, use ClientLogin and use the authorization token to obtain an app specific cookie which indicates the current user. This simple HTTP library can do this for you and all subsequent requests will use this cookie to tell the App Engine app who the current user is. Try it out by making the request to the simple user app that you visited earlier:

You can use the appengine_login method with your own app, just change the argument to the App ID of the app you want to access.

Along with simplifying access to apps which use Google Accounts, I wanted this library to simplify the process of using another feature used by many web apps: HTML form posts. Now I'm certain you've used HTML forms before, here's a simple example:

If you've even wondered what gets sent across the wire to post on a form like this, look back in your terminal to see the request from your computer and the response from the server (this is of course just the HTTP layer, wireshark will show you traffic on the IP and Ethernet layer as well).

That's really all there is to it. I designed this as just a simple script to use on the command line and I wrote it in less time than it's taken me to write this blog post about it (I borrowed atom.http_core from the gdata-python-client as a foundation). With some tweaks to remove the interactive (getpass and raw_input) calls and replace them with parameters, I could see this module as a utility layer in a larger, more complex, App Engine client application. If you're creating on I'd love to hear about it ;-)

For more information on how the appengine_login method works behind the scenes, see this presentation I gave a few months ago:

Many thanks to Trevor Johns and Nick Johnson for helping me to understand how this ClientLogin-to-cookie exchange works.

I'm sure that App Engine's Java runtime users would appreciate a port of this simple library to Java, if you feel so inclined.